Woolly aphids are much like regular aphids except these secrete a white waxy protective substance not unlike that secreted by scale insects and mealy bugs.
They love to cluster at joints and junctures of branches and stems.
By Rosser1954 , CC BY-SA 3.0, Link
Rarely do their colonies reach significant enough numbers but their waxy masses are unsightly on bonsai. They can cause leaves to curl and discolor and since they produce honeydew can also enable the growth of sooty mold.
Mostly these can just be wiped away using a cloth or a cotton swab dipped in methylated spirits or rubbing alcohol.
Systemic insecticides can be used for large run away infestations.
Systemic insecticides are taken up by the plant into its sap circulation system and thereby are take up by the aphids as they suck the plants sap.